1. Technical Field
The present invention relates to a method of controlling a controllable antenna included in a wireless transmission device, so as to enable the wireless transmission device to perform stable data transmission (transmitting and receiving) of application data depending on radio wave propagation environments.
2. Background Art
In a network configuration connecting information terminals to one another, wireless transmission devices have advantages of a terminal portability and a placement flexibility higher than those of wired transmission devices, and advantages of weight reduction without using wired cables, for example. The advantages allow the wireless transmission devices to be presently provided in various home appliances to transmit video and audio, in addition to conventional use in data transmission for personal computers.
While the wireless transmission devices have the above-described advantages, they also have disadvantages. The wireless transmission devices perform communication by emitting electromagnetic waves into space. Thereby, in space where many reflecting objects are placed, the transmission characteristics of the wireless transmission devices are often deteriorated due to influence of fading phenomenon caused by radio waves (delayed waves) propagated after having been reflected on the objects. In order to reduce the influence, there are methods including various diversity methods, methods of performing directional control on an antenna, methods employing an adaptive array antenna for weighting signals in signal processing, and the like.
FIG. 13 is a block diagram of a wireless transmission device that controls antennas using a so-called spatial diversity method. In the spatial diversity method, a plurality of antennas 901-1 to 901-N are arranged to be uncorrelated to one another as much as possible. Signals received by the antennas 901-1 to 901-N are provided to a radio-frequency switch 902. The radio-frequency switch 902 selects a single signal sequence from a plurality of input signal sequences, and provides the selected one to a radio-frequency processing unit 103. As a method for the selection, it is well known that signal levels of the plurality of signal sequences, or pieces of information such as a Bit Error Rate (BER) that are feedback from a physical layer processing unit 104 as described in Patent Reference 1, are compared to one another, so that a signal sequence having a maximum or minimum signal level (or information) is switched to be output. In addition, pieces of information that are feedback from a data link layer processing unit 105 are also used to select a signal sequence.
As shown in FIG. 13, the upper processing blocks including the physical layer processing unit 104 are standardized by a seven-layer model of Open Systems Interconnection (OSI). In packet communications via the Internet, five protocols of the physical layer, the data link layer, the network layer, the transport layer, and the application layer among the seven-layers are often used. Furthermore, as shown in FIG. 13, a transport layer processing unit 107 includes a buffer 108.
There is also a maximum ratio combining method as shown in FIG. 14, by which variable attenuators 1002-1 to 1002-N and variable phase shifters 1003-1 to 1003-N adjust amplitudes and phases, respectively, of the plurality of signal sequences to maximize a Signal to Noise power Ratio (SNR) of signals output from a weighting synthesis unit 1001 to the physical layer processing unit 104, and then an adder 1004 synthesizes the plurality of signal sequences.
In the maximum ratio combining method, weighting is performed depending on the amplitudes and the phases so that a synthesized directionality pattern generated by signal processing is, for example, null in a direction to which interference waves are propagated. Here, a device that temporally changes weight coefficients in accordance with radio wave propagation environments is called an adaptive array antenna
Patent Reference 2 discloses an antenna that includes a plurality of parasite elements in each of which a matched load is connected around a radiation element. By switching the matched load ON or OFF, a directionality of the antenna element is physically changed to optimize a parameter such as a Signal to Interference power Ratio (SIR). Thereby, the antenna performs control for deciding a combination of ON/OFF of a plurality of the matched loads.
While in the above methods the antenna control is performed based on indexes obtained in the physical layer or the data link layer, a method disclosed in Patent Reference 3 performs antenna control based on data throughput in the application layer.
Each of the methods attempts to control an antenna to be in an optimum transmitting/receiving state in accordance with radio wave propagation environments.